1. Field of the Invention
The present invention relates to apparatus for correcting direct current (DC) component of output voltage in an inverter, wherein automatic correction is performed in an inverter as a power converting system to convert DC voltage into alternating current (AC) voltage so that DC component of the output voltage is reduced.
2. Description of the Prior Art
In recent years, variable speed operation and control of motors are becoming more important as one basic technology to support daily life and industrial production. An inverter constitutes the center roll in the variable speed control technology and enables the variable speed operation of motors. The inverter is being actively utilized together with the rapid development of the power and microelectronics accompanied by the improvement of reliability, operability and maintainability of technology. One of matters to be considered in utilizing such inverter is a countermeasure regarding elimination of DC component or harmonic component contained in AC voltage which is converted and outputted by the inverter. Such DC component or harmonic component in AC voltage makes unstable the operating state of, for example, a motor being supplied with power, and deteriorates reliability of the operation control by the inverter.
An example of the above-mentioned inverter application technology is disclosed in "Conference Record of the 1986 IEEE Industry Applications Society Annual Meeting Part I" p.5l3 "Chargerless UPS Using Multi-functional BIMOS Inverter - Sinusoidal voltage waveform inverter with current minor loop - ". Main circuit constitution of a voltage type inverter shown in FIG. 1 of this reference is illustrated in a part of FIG. 1 in the accompanying drawings of the invention. In FIG. 1, an inverter main circuit M is connected in parallel between DC power source 1 such as battery and output terminals 16a, 16b; in FIG. 1 the inverter is shown surrounded by a dash-and-dot line. The inverter main circuit M comprises, in the order from the power source side, a capacitor 2, inverer arms 3a, 3b, 3c and 3d constituted by parallel connection of NPN type transistors TR1, TR2, TR3 and TR4 and diodes D1, D2, D3 and D4, a transformer 4 connected to output side of the inverter arms 3a-3d, a reactor 5 connected to secondary side of the transformer 4, and a capacitor 6 connected also to secondary side of the transformer 4 in parallel connection. An inverter control circuit C, shown surrounded by dash-and-dot line in FIG. 1 is connected to lower side of the inverter main circuit M in FIG. 1. The inverter control circuit C comprises a voltage detector 7 such as a potential dividing resistor, an oscillator 8 for determining output frequency of the inverter main circuit M, a setting device 9 of voltage command value given from outside for determining output voltage value of the inverter main circuit M, a sinusoidal wave generator 10 for generating a sinusoidal wave as output voltage reference based on both the frequency set by the oscillator 8 and the voltage command value set by the voltage command value setting device 9, an adder-subtractor 11 for adding the signal produced by the polarity inversion of the output of the voltage detector 7 and the output signal of the sinusoidal wave generator 10, an error amplifier 12 connected to output side of the adder-subtractor 11, a triangular wave (carrier) oscillator 13 for determining the switching frequency of the arms 3a, 3b, 3c and 3d of the inverter main circuit M, a comparator 14 for comparing respective outputs of the error amplifier 12 and the triangular wave oscillator 13 and for generating ON/OFF signal for each of the transistors in the arms 3a, 3b, 3c and 3d, and a transistor drive circuit 15 for driving the transistors TR1-TR4 to constitute the pattern arms 3a -3d in response to the output signal of the comparator.
Operation of the inverter in the above-mentioned constitution will now be described. The transistors TR1, TR2, TR3 and TR4 constitute arms of so-called single phase bridge inverter, and have the basic function of converting DC voltage from the DC power source 1 into AC voltage. Assume that one AC terminal to which emitter of the transistor TR1 and collector of the transistor TR2 are connected is made U, and other terminal to which emitter of the transistor TR3 and collector of the transistor TR4 are connected is made V. While the arm 3a and the arm 3d are simultaneously turned on, the arm 3b and the arm 3c are turned off. Then, positive potential is applied to the terminal U, and negative potential is applied to the terminal V. In reverse, when the arms 3b and 3c are turned on and the arms 3a and 3d are turned off, negative potential is applied to the terminal U and positive potential is applied to the terminal V.
Switching of respective arms is performed at a frequency several times higher than AC output frequency (e.g., commercial frequency) of the inverter and moreover the conduction time of each arm is set to a suitable time and controlled; thereby a prescribed inverter output voltage can be obtained. Such an inverter is generally called an inverter of a pulse width modulation (PWM) system. The capacitor 2 is a filter to remove harmonic ripple current flowing through the DC circuit, and the reactor 5 and the capacitor 6 constitute a filter to remove harmonic component contained in the AC output voltage. Further, the diodes D1-D4 become conductive when regenerative current in each arm flows from AC side to DC side, and are connected in parallel to each transistor in the reverse direction to the conductive direction of each transistor. Output voltage of the inverter main circuit M is transformed by the transformer 4 into a suitable voltage for various loads (not shown).
On the other hand, the inverter control circuit C constitutes PWM control circuit in a triangular wave comparison system, and AC output voltage of the inverter is controlled so that its frequency is determined by the oscillator 8 and amount of the voltage is determined by the voltage command value from outside. The sinusoidal wave generator 10 generates a sinusoidal wave signal whose frequency is determined by the oscillator 8 and amount of the voltage is determined by the voltage command value set by the voltage command value setting device 9. The voltage detector 7 is constituted by potential dividing resistor or the like as above described, and when output voltage of the inverter is supplied to the control circuit C the output voltage is divided and a suitable voltage level and output voltage signal are generated. The adder-subtractor 11 subtracts the output voltage signal from the sinusoidal wave signal and generates an error signal. The error amplifier 12 amplifies the error signal by suitable transfer function and generates a voltage reference signal.
The triangular wave oscillator 13 oscillates a reference triangular wave voltage signal to determine switching frequency and switching time-point of the transistors in the arms 3a,3b,3c and 3d . The comparator 14 compares the voltage reference signal and the reference triangular wave signal in amount, and further performs correction with shortcircuit prevention time so that the arms in series connection, more specifically two series bodies of the transistors TR1, TR2 and the transistors TR3, TR4 are not simultaneously rendered conductive, and generates the switching command signal. The switching command signal is insulated by a photo coupler or the like installed in the drive circuit 15, and further amplified to a current having sufficient magnitude to perform ON/OFF operation of the transistors and supplied as base current to each transistor. Thus the inverter converts DC voltage from the DC source 1 into AC voltage controlled by the command value of the voltage command value setting device 9 and the frequency of the oscillator 8, and outputs the AC voltage to the output terminals 16a, 16b.
Since the apparatus for correcting the DC component of the output voltage in the inverter of the prior art is constituted as above described, the AC output voltage inevitably contains a DC component due to variation of the voltage drop of each arm of the inverter main circuit or variation of conduction time of the transistors. For example, if sum of the voltage drop of the transistors TR1 and TR4 (collector-emitter voltage of transistors during conduction) is less than the sum of the voltage drop of the transistors TR2 and TR3, it follows that the AC output voltage of the inverter contains DC voltage of positive polarity even if the conduction time of respective arms is equal in symmetry. On the other hand, if the conduction time of the transistors TR1 and TR4 is longer than the conduction time of the transistors TR2 and TR3 due to difference in characteristics of the transistor drive circuit 15, DC voltage of positive polarity is superposed to the AC output voltage also in this case.
A problem exists also in that the above-mentioned DC component causes DC eccentric magnetization of the transformer as load connected to output side of the inverter main circuit and excessive current flows. Since such DC component is generated due to variation in characteristics of the transistor itself or of the transistor drive circuit or the like, when the amount of the output voltage of the inverter is varied or a transistor is exchanged, the DC component appearing in the AC output varies and therefore it is difficult to decrease the DC component only by adjusting the inverter control circuit.
Also in the inverter main circuit in the prior art, in order that the DC component is not outputted, the transformer 4 is connected between the output terminals 16a, 16b and a gap is provided in the core of the transformer 4 and a special leakage transformer must be used so that the exciting current becomes larger than current flowing due to the DC voltage contained in the inverter output voltage. Consequently, a relatively cheap transformer having ordinary characteristics cannot be used. Even if the voltage need not be transformed, the inverter device cannot be constituted without using the transformer.